The present invention relates generally to data communications between electronic circuits. More particularly, the invention relates to a method for permitting simultaneous, multi-directional communications between a number of different circuits using a common transmission line.
Electronic systems commonly included many different electronic circuits which must communicate data with each other through suitable data transmission lines or networks. A simple data communication arrangement includes a separate transmission line for each individual signal which must be communicated from one circuit to another. However, as integrated circuit technology has advanced, far more circuitry can be placed on an integrated circuit chip than can be accessed externally through separate signal transmission lines. An integrated circuit package will inevitably have room for fewer input/output pins than the number of signals which must be communicated to a separate integrated circuit or received from a separate integrated circuit.
Time division multiplexing may be used the allow different signals to effectively share a common transmission line. In time division multiplexing, a single transmission line carries one signal at one time and another signal at another time. However, time division multiplexing adds complexity to the circuit. Furthermore, since signals are not communicated simultaneously, time division multiplexing reduces the overall speed of data communications.
U.S. Pat. No. 5,761,246 discloses a circuit and signal transmission method in which a number of different signals may be communicated from one circuit to another simultaneously over a single signal transmission line. However, the arrangement shown in the U.S. Pat. No. 5,761,246 facilitates only unidirectional signal transmission. That is, although multiple signals from one circuit are communicated simultaneously to a second circuit over a single transmission line, a separate transmission line was required for transmitting signals from the second circuit back to the first circuit.
U.S. Pat. No. 4,604,740 discloses a digital transceiver allowing full duplex or bi-directional transmission of digital signals over a common transmission line. This transceiver is limited to only bi-directional transmission between two circuits, and does not facilitate multi-directional communications between three or more circuits.
It is an object of the invention to provide a data transmission method which enables multiple digital signals to be communicated simultaneously in multiple directions between circuits connected by a single transmission line or network. The method allows each circuit in the arrangement to transmit and receive digital data at the same time each other circuit transmits and receives digital data, all using the single signal transmission line.
The object of the invention is accomplished by using digital signals from a group of three or more circuits to create an encoded or combined signal on a common transmission line, and then decoding the encoded signal at each different circuit to produce or recreate the digital signal asserted by each different circuit in the group. The encoded signal comprises a signal included in a set of unique signal values, with each signal in the set corresponding to a different combination of digital signals asserted by the group of circuits. Decoding the encoded signal at each circuit in the group involves comparing the encoded signal to a particular reference voltage from a set of reference voltages. A particular reference voltage used in this comparison may be selected using one or more digital signals already decoded from the encoded signal.
The method according to the invention employs an encoding and decoding arrangement with each separate circuit in a system of circuits. Encoding devices included with the separate circuits cooperate to produce the encoded signal on a common transmission line or network which interconnects the various circuits. The decoding arrangement associated with each circuit receives the encoded signal appearing on the transmission line and decodes the encoded signal to provide the digital data signals transmitted from the other circuits in the system. For example, a system or group of circuits according to the invention may include a first circuit producing a first data signal, a second circuit producing a second data signal, and a third circuit producing a third data signal. In this example the first circuit includes a first circuit decoding arrangement for decoding the encoded signal to produce or recreate the data signals transmitted from the second and third circuits. The second circuit includes a second circuit decoding arrangement for decoding the encoded signal to produce or recreate the data signals from the first and third circuits. Finally, the third- circuit includes a third circuit decoding arrangement for decoding the encoded signal to produce or recreate the data signals from the first and second circuits.
The encoding arrangement includes a resistor in each circuit connected between a signal driver for the respective circuit and the common signal transmission line. These resistors cooperate to produce the encoded signal at a particular voltage level. This encoded signal is representative of a particular combination of digital data signals transmitted by the various circuits in the system. In the example system including three separate circuits each producing a single digital signal, the encoded signal will comprise one signal from a set of eight unique voltage level signals. Each voltage level in the set of signals represents a different combination of digital signals transmitted by each of the circuits in the multi-directional data transmission system.
Decoding the first digital signal transmitted from a first circuit in the system is performed by comparing the encoded signal on the common transmission line to a first reference voltage. Decoding a second digital signal from a second circuit in the system is performed by comparing the encoded signal on the common transmission line to a decoding reference voltage comprising one of two reference voltages depending upon the value of the first digital signal. The first digital signal may be used to control a multiplexer to pass the correct reference voltage for comparison. Decoding a third digital signal from a third circuit in the system is performed by comparing the encoded signal on the common transmission line to another decoding reference signal comprising one of four reference voltages depending upon the value of the first and second digital signals. An additional multiplexer under the control of the first and second digital signals may be used to pass the correct reference voltage for use in decoding the third digital signal. In each case, the comparison is preferably performed by a differential receiver having a positive input connected to receive the encoded signal on the common transmission line and a negative input connected to receive the appropriate reference voltage.
Each circuit in the multi-directional data communication system includes only the decoding structure required for decoding the signals transmitted from the other circuits in the system. In the three circuit example, the first circuit includes a first circuit decoding arrangement comprising the multiplexers and differential receivers necessary to decode the second and third signals. The second circuit includes a second circuit decoding arrangement comprising the differential receivers and the additional multiplexer required to decode the first and second digital signals. Finally, the third circuit includes a third circuit decoding arrangement comprising the differential receivers and the multiplexer required to decode the first and second signals.
The present invention is not limited to the three circuit example set out here for purposes of describing the invention. Rather the concepts of the present invention may be employed in multiplexing N signals on a common transmission line, where N is a positive integer greater than two. The value of N is generally limited by the quality of the differential receivers and the voltage level of the digital signals employed in the circuit system.
The present invention reduces the number of input/output pins on an integrated circuit package required to interconnect the integrated circuits for data communication. Also, the invention reduces the number of transmission lines required for simultaneous, multi-directional data communications between different circuits. Furthermore, by allowing simultaneous data communications between circuits, the invention may increase overall system performance.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.